XDC Developers' Handbook


XDC is an application which allows the generation of documentation pages in HTML format from special comments ("XDC" comments) within XML files of various dialects.

XDC strongly resembles the Javadoc tool included in the JDK of the Sun Java Platform Standard Edition (JavaSE). This resemblance is reflected

- in the format of the comments:

Like Javadoc comments, which differ from standard multi-line comments in Java source files by a additional asterisk ("*") marking the starting delimiter of the comment, XDC comments differ from standard XML comments by an additional dash ("-") character opening the comment:

<!-- This is a standard XML comment -->

<!--- This is an XDC comment -->

- in the way the tool is invoked:

Like the Javadoc tool, XDC can be invoked on the command line with an almost identical format. Moreover, most options that XDC recognizes are identical to those of Javadoc.

The XDC tool can also be invoked in a build process controlled by an Apache Ant build file. For this, a xdc task is included in the XDC distribution. This task has again a similar set of attributes and recognized subelements as the javadoc task.

- in the format of the generated HTML pages

The look and feel of the HTML pages generated by the XDC has been designed to also resemble the files produced by the Javadoc tool.


The DJDoc API is licensed under the Apache Software License 2.0.

The application uses the following APIs which are included in the distribution:
Archive(s) Description Version Use License Remarks/Resource URL
log4j.jar Logging library 1.2.13 compile,runtime Apache Software License 2.0 http://logging.apache.org/log4j/
commons-cli-1.0.jar Commons CLI (Command line interface) 1.0 compile,runtime Apache Software License 2.0 http://jakarta.apache.org/commons/cli/
xalan.jar Xalan XSLT transformer 2.7.0 compile,runtime Apache Software License 2.0 http://xml.apache.org/xalan-j/
xercesImpl.jar, xml-apis.jar, serializer.jar Xerces XML parser 2.6.2 runtime Apache Software License 2.0 http://xerces.apache.org/xerces2-j/
ant.jar Apache Ant build framework 1.6.5 compile,runtime Apache Software License 2.0 http://ant.apache.org/
nalax.jar Xalan extension package 0.9.4 runtime Apache Software License 2.0 http://nalax.sourceforge.net
junit.jar Java unit testing framework 3.8.1 compiling (just unit tests), testing Common Public License 1.0 http://junit.sourceforge.net/
alster.jar XSLT unit testing framework 0.5.1 testing Apache Software License 2.0 http://alster.sourceforge.net


Installation from Source

If you have downloaded the XDC sources, follow these steps to install the software:

  1. Extract the source archive into a new directory and open a shell in that directory.

  2. Run the build.sh resp. build.bat script. This will invoke the Ant build utility and execute a number of build steps such as cleaning the directories, compiling the XDC and the unit test sources, running the tests, generating the documentation and packaging the generated classes into a JAR file.

Binary Installation

If you have just downloaded the binaries of the XDC application, install XDC by following these steps:

  1. Extract the XDC binary archive into a new directory.

The XDC tool can now be run by executing the xdc.sh resp. xdc.bat scripts. If you run these in a directory other than the XDC root directory, make sure that the xdc.jar archive and all JAR files in the /lib directory are in the classpath.


Command Line

The XDC application can be invoked from the command line using the class net.sf.xdc.Main as the entry point. The processing of the command line arguments is handled by the Commons CLI (command line interface) library of the Apache Jakarta project (somewhat extended to meet special needs).

For easier invocation of the tool, two shell scripts (xdc.bat resp. xdc.sh) have been provided which take care of correctly setting the classpath, the Java VM options and the actual invocation of the Java runtime.

HTML File Generation

Almost all of the files produced by the XDC tools are generated using XSLT transformations. The heart of the XDC application consists of a hierarchy of XSLT stylesheets responsible for converting XDC documentation as well as some information about the structure of the documented XML files into various parts of the generated documentation.

CSS Stylesheet Files

The default CSS stylesheet file is not produced from an XML source but is simply copied to the correct location in the output directory hierarchy.

Other Support Files

The remaining support files are generated from various XML sources:

  • The help file is generated from a source in docbook format (net/sf/xdc/resources/help-doc.xml)) using a proprietary docbook stylesheet (net/sf/xdc/xsl/docbook.xsl).

  • Depending on the number of packages involved in the XDC invocation, the correct frameset is generated from an empty XML document (plus some global stylesheet variables) using one of the net/sf/xdc/xsl/index?.xsl stylesheets.

  • The lists of documented packages and/or files are produced from XML files containing the corresponding lists (generated by the Java class net.sf.xdc.processing.XdcSourceCollector) using one of the net/sf/xdc/xsl/*-frame.xsl stylesheets.

Summary Files

The XDC tool produces one package summary file which contains a table with summary data for each file documented within that package, along with HTML code contained in a file named xdc-package.html. Similarly, if XDC documents more than one package, an overview file is generated with a summary table for each documented package as well as HTML code read from a file specified by the -overview option.

Source Element Pages

The XDC tool produces one file for every documented XML file. The XSLT stylesheet used for the transformation of the source XML file is determined by the XML dialect of the source file. XDC includes three built in dialect-specific XSLT files named ant.xsl, xsl.xsl and generic.xsl used for Ant build files, XSLT stylesheet files and generic XML files, respectively.

XML source view

If the -linksource option is used, the XDC tool produces an additional HTML presentation of each XML source file in a separate folder. This file is produced using an instance of the net.sf.xdc.processing.SourceProcessor class by parsing the content of the source files line by line and streaming it into the output file along with line numbers and anchors (and with the special XML characters appropriately escaped).

Standard Compliance

In the design phase of the XDC tool, it appeared quite natural to use XSLT stylesheets for the "engine" of the application because XSLT is the prime choice when it comes to converting XML to HTML.

The goal of the implementation was to use pure XSLT 1.0 in the stylesheets. This had the big advantage that the transformation of the source XML file to HTML (which really made up the heart of the application) could be done not only by the XSLT transformer used by the Java runtime used but also by any XSLT 1.0 compliant transformer. In particular, the XSLT transformers used by the common web browsers Micorsoft Internet Explorer (MSXML) and Mozilla Firefox (Transformiix) could be used for "previewing" the XML source files (e.g. by including a processing instruction like <?xml-stylesheet type="text/xsl" href="generic.xsl"?> in the XML file). This functionality has proved very valuable especially for "debugging".

The only exception to the compliance to the XSLT 1.0 standard are the use of the java:org.apache.xalan.lib.NodeInfo.lineNumber() function in two places: One place this is used is for writing an <xsl:message> with the line number of an XML subelement which fails to provide XDC documentation (in case the -reportmissing option has been selected). The other place is in the hyperlink from the detail view of an XML subelement to the corresponding anchor in the XML source view page (for which there was really no alternative). As a consequence of this deviation from the standard compliance, the preview functionality fails to provide a correct hyperlink to the XML source view page.

During the development phase, a few drawbacks of the "pure XSLT 1.0" approach became apparent:

Future enhancements

There are a number of features that a followup version of XDC may include.

Integration with external API dcumentation

Support for the Javadoc -link and -linkoffline options should also be added in the future. An integration with Javadoc generated API documentation should also be possible.

Enhanced extendability

Another feature probably implemented in a future version of XDC will be the equivalent of the Javadoc Doclets and Taglets which will give the user more flexibility in the presentation of the generated documentation as well as control over how certain (dialect-specific) tags are handled.

Alternative XDC comment format

As mentioned above, a followup version of XDC might allow XDC information to be placed in XML subelements from a custom namespace besides the option to use XDC comments.

Support for "inclusions"

Each of the two main target XML dialects of the XDC application, Apache Ant scripts and XSLT stylesheets, has ways to include other XML files (Ant does so with the <import> element; XSLT with the <xsl:import> and <xsl:include> elements). Moreover, any XML file has the possibility to include another file as an external entity.

A future version of XDC is likely to have some sort of support for all these types of inclusions (and some of its consequenses such as Ant target overriding or XSLT template precedence) as Javadoc has for Java inheritance related features.

Partial rearchitecture

The more awkward the development in "pure XSLT 1.0" mode becomes, the greater the urge will be to open up this concept and solve particular problems with custom XSLT extensions - even if that means that XDC would be restricted to the Xalan XSLT processor and would thus lose the preview functionality.

Also, the XDC architecture may move away from the concept which identifies one source file with one documented page and one source directory with one package. Depending on the complexity of the documented sources, it might instead be desirable to split up one source XML file into several pages in the generated documentation (somewhat similar to what Javadoc does with inner classes) or combine several source XML files to one page in the documentation (comparable perhaps to the way TLDDoc/taglibrarydoc does).

Tool integration

Finally, it would be nice to have support for different aspects of the XDC product in IDEs or other tools. This could include custom syntax highlighting of XDC comments, automatic comment skeleton generation in XML files, code inspections (issuing warnings in cases of missing or incomplete XDC comments) and XDC comment previewing.

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